The invention relates on the one hand to a process for preparing alkali- and heat-stable sugar alcohol compositions which exhibits an optical density lower than or equal to 0.100 in an S-test. On the other hand, the invention relates to sorbitol compositions.
Alkali- and heat-stability of sugar-alcohols is important in all those applications where colour formation under these conditions must be prohibited. This is the case, e.g. where polyol compositions are used as humectants in tooth-pastes containing alkaline abrasives, as building blocks of polyether polyols, or as starters for preparing sorbitan esters. Colouring of end-products containing these polyols is often due to the presence of colour-forming precursors, including residual reducing sugars, in the sugar-alcohol compositions used.
This problem is quite well known and a number of solutions have already been proposed to improve alkali- and heat-stability of such polyol compositions.
In JP 63079844, a method is described in which an aqueous sugar alcohol solution is adjusted to a pH-value of between 8 and 13, followed by a (discontinuous) heating step at temperatures varying between 90° C. and 220° C. The resulting product is then purified by passing the polyol solution through a strongly acidic cation exchange resin, a strong base anion exchange resin and a mixed bed resin.
In EP 0 711 743, a similar process is disclosed in which the polyol composition is first stabilised by means of an oxidation, a fermentation or a caramelisation step, followed by a purification of the solution. The purification step is comparable to the one disclosed in JP 63079844.
In EP 1 095 925, a purification process is disclosed comprising a first treatment on a strong acid cationic exchange resin at a temperature below 50° C., preferably below 40° C., followed by a treatment on a strong basic anionic resin and a mixed bed resin.
Methods for removing aldehydes and other reactive impurities, and/or stabilising colour in glycerol and glycol aqueous solutions have been discussed in FR 1 546 472 and U.S. Pat. No. 6,187,973 respectively. In both cases, a strong base anion exchange resin in the hydroxide form was converted into the bisulphite form, in order to treat the aqueous polyol solutions.
The major disadvantage of the above-cited processes resides in their complexity. Indeed, first a chemical stabilisation step is needed, followed by a quite complicated purification step. During this stabilisation step, high pH-values are used. In addition, it is necessary to use quite high temperatures (>90° C.) and long reaction times to obtain the necessary stabilisation.( see JP63079844 and EP711743). This results in a rather important chemicals consumption during the chemical reaction, and later on, for the regeneration of the different ion exchange resins.
In addition, two separate steps are needed to arrive at the desired result. The equipment needed, therefore comprises a reactor to perform the stabilisation step, and at least two ion exchange resin batteries to perform the purification step.
In the case of the bisulphite-type resins, the use thereof proves to be inefficient when higher polyols such as pentitols, hexitols and/or hydrogenated starch hydrolysates are treated in the expectation of obtaining alkali- and heat stable products.